The present invention relates to offshore jack-up oil drilling and production platforms and more particularly to a device for overloading this type of platform.
The platforms of this type usually comprise legs which bear against the sea bed and a hull which is movable and adjustable in height along the legs.
The hull of the platform is brought in a floating state to the drilling or production site and the legs are lowered into contact with the sea bed. Then, by bearing against the legs, the hull is jacked up above the level of the sea at a height which puts it out of reach of the highest waves.
The hull is therefore movable along the legs of the platform by jack mechanisms disposed in a carrying structure which is well known to those skilled in the art by the name "Jack-house". These jack mechanisms comprise output gear pinions cooperating with racks mounted on at least a part of the length of the legs. These pinions are driven by a plurality of motors associated with speed reducers whose speed reduction ratio is very high.
Further, each leg is provided in its lower part with a foot, for example of hexagonal shape, which bears against the sea bed.
When laying the legs of the platform on the sea bed, safety standards require the operators to apply on the legs an overload which may be as much as 1.25 times the load of the platform. This overload has for its purpose to achieve a stable anchoring of the legs, especially on sites where the feet have a tendency to sink in due to the instability of some layers of the subsoil.
For this purpose, the hull of the platform is provided with, in addition to the exploitation equipment and living quarters, chambers integrated with the hull which may be filled with sea water when laying the legs on the sea bed. In this way there is created an overload which is transmitted to the legs through the jack mechanisms.
However, safety standards are becoming increasingly severe and consequently the overload required to be applied to the legs is increasing.
One solution would consist in increasing the number of the chambers in the hull of the platform.
Now, as the available space in the hull is limited, it would be necessary to increase the dimensions of the hull. This would result in greater weight and higher drag forces when towing the platform.
Further, this arrangement would subject the jack mechanisms and in particular the output pinions to a high load which would require increasing the size of these pinions.